The present invention relates to an apparatus for controlling the amount of secondary air fed into an intake passage or into an exhaust passage of an internal combustion engine for controlling an equivalent air-fuel ratio (if an air-fuel passage from the intake passage through exhaust passage located upstream of an air-fuel ratio sensor is defined as a working fluid passage, the equivalent air-fuel ratio is defined as a ratio of the amount of air fed into the working fluid passage to the amount of fuel fed into the working fluid passage) within a predetermined range.
In the field of this art, a method is known in which the equivalent air-fuel ratio is detected by an air-fuel ratio sensor, for example, an oxygen concentration sensor for detecting the concentration of the oxygen component in the exhaust gas, and; then, secondary air is fed into an intake passage or into an exhaust passage of an internal combustion engine according to the detected equivalent air-fuel ratio, for maintaining the equivalent air-fuel ratio within a predetermined range which is near the stoichiometric air-fuel ratio, whereby the effect of purifying pollutants in a three-way catalytic converter disposed in the exhaust system is improved.
In a conventional apparatus for carrying out the above-mentioned method, the amount of secondary air to be injected into the engine is controlled by an air flow control valve disposed in a passage between an air pump and a secondary air injection mechanism. The air flow control valve is driven by an actuating pressure applied thereto through an electromagnetic valve which is adapted for switching the transmission of the actuating pressure on or off in response to an electrical signal provided from the air-fuel ratio sensor. More specifically, when a lean signal, which indicates that the equivalent air-fuel ratio is on the lean side of the stoichiometric air-fuel ratio, is provided from the air-fuel ratio sensor, a diaphragm of the air flow control valve is not actuated by the actuating pressure and is pressed by a return spring, so as to form a passage for discharging the air fed from the air pump into the atmosphere. Furthermore, when a rich signal, which indicates that the equivalent air-fuel ratio is on the rich side of the stoichiometric air-fuel ratio, is provided from the air-fuel ratio sensor, the diaphragm of the air flow control valve is actuated by the actuating pressure against the pressing force of the return spring, so as to form a passage for providing the air fed from the air pump to the secondary air injection mechanism.
The actuating pressure may be a negative pressure, such as a vacuum pressure provided from an intake manifold of the engine, or a positive pressure, such as discharge pressure of the air pump.
However, in the conventional apparatus of the above described type, the electromagnetic valve is driven corresponding to the electrical signal provided from the air-fuel ratio sensor to control the level of the actuating pressure and, then, the air flow control valve is driven in accordance with the actuating pressure to control the amount of secondary air fed into the engine. Thus, the amount of secondary air is controlled regardless of the amount of the exhaust gas flowing per unit time; in other words, regardless of the speed of the flow of the exhaust gas.
Therefore, when a rapid increase of the exhaust gas flow occurs, in other words, when the engine is rapidly accelerated, the valve controlling the amount of secondary air fed into the engine cannot respond to such changes of the engine conditions. As a result, it is very difficult to control the equivalent air-fuel ratio within the predetermined range during such transition periods.
In order to improve the response speed of the air flow control valve during transitional engine conditions, there exists a conventional method for instantly increasing or decreasing the actuating pressure applied to the air flow control valve when the level of the electrical signal is changed. However, such method sometimes causes excessive control, in other words overcontrol, of the equivalent air-fuel ratio every time the air flow control valve is switched.